Basically, have the split window thing, but the top window (or wherever you specify...possibly be able to resize/move it with /window or something) is all the joins,quits,parts,nickchanges,etc from the channel in the bottom window, and the bottom window is just chat (and have a join/part/quit window individually for each channel, not 1 global join/part/quit window).

1. a "addon" to /filter such as: /filter -window add joinquit * irc_join,irc_part,irc_quit *
(note the -window switch, that would be the key for the functionality, then be able to resize/move/etc it with /window).

2. It's own command, something like: /noticewindow (not the best name for it, i admit) that automagically opens the window, it only works for joins/parts/quits/nickchanges and you use /noticewindow resize 20% (for example) to resize the window with that stuff, or /noticewindow move bottom to move that window to the bottom and have chat on top.

3. a /set option: something like: /set irc.look.join_part_split on/off that would create or destroy the window (whether it's set on or off) and again, it's resized/moved by /window.

4. A script that would either create the buffer on load and manage it...somehow or implement #2 or #3, but as a script. Doing this as a script would probably require some API additions.